Lateral movement of a laser beam is typically accomplished in one of two fashions. In one approach, a laser beam is steered by a mirror capable of angular movement. For example, in FIG. 1, a laser source 10 is fixed in its position to direct laser beam 12 towards a galvo-mirror driver 14. As is known in the art, driver 14 is a motor-like device with a mirror affixed to its shaft. The shaft can make small angular movements to either side of a given center position. Thus, driver 14 operates to change the angular position of laser beam 12 relative to a workpiece 16. However, the angular movement of driver 14 must be controlled very precisely in order to achieve specific lateral displacement (e.g., in the x and/or y directions) on the surface of workpiece 16. In addition, each time the angle of laser beam 12 relative to workpiece 16 changes, so does the path length of the beam. This can present problems in laser machining operations (e.g., custom corneal ablation) where the laser beam may have been optimized for a particular path length. Thus, a change in path length means that the laser beam is no longer optimized when it strikes the workpiece. This can result in either too little or too much "machining" at the surface of the workpiece.
The second approach, shown schematically in FIG. 2, involves the use of a mirror 20 and a linear positioner 22 coupled to the mirror. A laser source 24 is fixed in its position and mirror 20 is placed in the path of the generated laser beam 26 at angle .theta. (e-g., 45.degree.) therewith. Linear positioner 22 is used to move mirror 20 linearly towards or away from laser source 24 as represented by two-headed arrow 28. Such movement of mirror 20 causes a 1:1 corresponding amount of lateral displacement of beam 26 at the surface of workpiece 30. While precise amounts of movement can be achieved with linear positioners, the path length of beam 26 from laser source 24 to workpiece 30 changes each time mirror 20 is moved. In addition, since beam displacement is in 1:1 correspondence with mirror displacement, response time in terms of beam displacement is dictated by the speed of the linear positioner.